The present invention relates to a process for reforming methane and higher hydrocarbons to produce a synthesis gas (syngas) product and/or a hydrogen product. A synthesis gas product is a product comprising primarily CO and H2. Reformed hydrocarbons may be further reacted in one or more shift reactors to form additional H2 in the process stream and separated in a separation unit, such as a pressure swing adsorption unit, to form a H2 product.
Synthesis gas is conventionally used to produce synthesis gas products such as synthetic crude, or further upgraded to form intermediate or end products, for example lubricant basestock, lubricants, greases, middle distillate fuels, diesel, linear alkylbenzenes aviation and jet fuels, gasoline, and other chemicals, i.e. normal- and iso-paraffinic solvents. The synthesis gas may also be used to produce one or more oxygenates, for example, ethers and/or alcohols.
Synthesis gas (including hydrogen) can be produced from methane-containing feedstocks by any number of primary synthesis gas generation reactors. For example, synthesis gas can be produced in a steam methane reformer (SMR), an endothermic reactor where reaction is carried out either in heat exchange reactors, or by other means where substantial heat may be transferred to the reacting fluid, such as in the case of autothermal reforming (ATR), where a portion of the feedstock is combusted inside the reactor to provide heat for steam reforming either subsequently or in the same location as the combustion.
Various feedstocks can be used to produce synthesis gas and industry desires the capability to process multiple feedstocks. Industry desires the flexibility to switch between various hydrocarbon feedstocks, such as natural gas, liquefied petroleum gas (LPG), naphtha, or mixtures thereof, due to changing availability and relative cost of each hydrocarbon feedstock. Industry desires the ability to switch from one feedstock to another during operation without shutting down the reactor. For example, a synthesis gas producer may desire to use natural gas for 6 months, naphtha for 3 months, and then a mixture of natural gas and naphtha for 2 months. Industry desires to be able to process any available hydrocarbon feedstock, while maintaining high energy efficiency, avoiding overheating, and avoiding carbon formation in the primary synthesis gas reactor.
In addition to being able to process multiple feedstocks, industry desires to be able to process a feedstock where the composition, particularly the C2+ hydrocarbon concentration in the feedstock, varies over time. For example, synthesis gas may be produced from a refinery offgas where the C2+ hydrocarbon concentration varies from 2 vol. % to 15 vol. % depending on the refinery operation.
If the feedstock contains higher hydrocarbons than methane, that is, hydrocarbons having 2 or more carbon atoms (C2+ hydrocarbons) are used in the reforming process, the risk for catalyst deactivation by carbon deposition in the primary synthesis gas generation reactor is increased. Industry desires to avoid carbon formation in the synthesis gas generation reactor.
In order to reduce the risk of carbon deposition in the primary synthesis gas generation reactor, hydrogen and synthesis gas production processes may employ at least one catalytic reactor prior to the primary synthesis gas generation reactor where the catalytic reactor is operated at conditions less prone to hydrocarbon cracking than the primary synthesis gas generation reactor. These reactors positioned before the primary synthesis gas generation reactors are referred to as pre-reformers. Pre-reformers can be operated adiabatically or convectively heated by indirect heat transfer with combustion products gases from the primary synthesis gas generation reactor.
A pre-reformer can provide several benefits. A pre-reformer can improve the energy efficiency of the overall reforming process (for example by allowing a lower steam-to-carbon molar ratio for the feed), reduce the capital cost of the primary reformer, allow for higher preheat temperatures to the primary reformer without carbon formation on the catalyst in the catalyst-containing reformer tubes, and reduce involuntary steam production.
In hydrogen and synthesis gas production processes employing pre-reformers and steam methane reformers, the hydrocarbon feedstock may be mixed with hydrogen for a resultant stream having 1 to 5 mole % hydrogen, and subsequently subjected to a hydrodesulphurization (HDS) pretreatment to remove sulphur. The hydrocarbon feedstock may also be treated to remove olefins in a hydrogenation reactor. In case H2 is present in the feedstock, additional H2 might not be added.
For steam reforming of heavy naphthas, hydrogen concentrations as high as about 50 mole % H2 are known where the mixture is subsequently pretreated in a hydrodesulphurization unit and/or a hydrogenation reactor. Even higher hydrogen concentrations are possible depending on the feedstock provided.
The feedstock, after pretreating, is combined with superheated steam to form “mixed feed” having a prescribed steam-to-carbon molar ratio. The steam-to-carbon molar ratio, S/C, is the ratio of the molar flow rate of steam in the mixed feed to the molar flow rate of hydrocarbon-based carbon in the mixed feed. The “steam-to-carbon molar ratio” is a conventional term used in the art.
The steam-to-carbon molar ratio for steam methane reforming of natural gas typically ranges from 2 to 5, but can be as low as 1.5. The steam-to-carbon molar ratio is generally higher for steam methane reforming of feedstock containing a greater amount of higher hydrocarbons, for example, propane, butane, propane/butane mixtures, and naphtha.
Higher steam flow rates are used to suppress carbon formation and enhance the steam reforming reaction. However, higher steam-to-carbon molar ratios disadvantageously decrease the energy efficiency of the reforming process. Industry desires to improve the energy efficiency of steam-hydrocarbon reforming systems.
While there are significant benefits when using a prereformer, it is known that when the feed to the prereformer contains a significant amount of higher hydrocarbons, as for example when naphtha is used to form the feed, the reaction in the prereformer is exothermic, while when the feed to the prereformer contains few higher hydrocarbons, as for example when natural gas is used to form the feed to the prereformer, the reaction in the prereformer is endothermic.
When processing feeds that result in an exothermic reaction in the prereformer there is a risk of overheating downstream heating coils, overheating the reactor (reformer) inlet, overheating the reactor (reformer), and cracking the feed thereby forming solid carbon. Additional steam can be added to the feed to reduce the risk of overheating and solid formation, but this reduces the efficiency of the overall process.
Industry desires to avoid overheating the prereformer reactor while maintaining high energy efficiency.